1. Field of the Invention
The present invention relates to a method for the compensation of charges in secondary ion mass spectrometry (SIMS) of specimens exhibiting poor electrical conductivity using a scanning ion gun and an electron gun, and also relates to an apparatus for carrying out the method in which the ion gun and the electron gun share a common scan generator.
2. Description of the Prior Art
In the analysis technique of secondary ion mass spectrometry (SIMS) the specimen to be investigated with respect to its elemental composition is bombarded with an ion beam (for example, oxygen O.sub.2.sup.+, 1-15 keV, 1 nA--10 .mu.A) and is thereby slowly eroded (a sputtering process). The atoms and molecule fragments thereby released (sputtered off) from the specimen surface can, insofar as they are electrically charged ("secondary ions"), be guided into a mass spectrometer, be separated there according to their mass-charge ratio m/e and be detected and counted with the assistance of a multiplier or the like.
In order to achieve a uniform erosion of the specimens over the area to be analyzed, the ion beam is generally focused onto the specimen surface and is scanned line-by-line across the area that is to be eroded on the specimen.
Given specimens having sufficient electrical conductivity, the specimen current deriving from the ion beam current and the charged secondary particle currents emitted from the specimen flows off to ground via the specimen support and the voltage source. The specimen support can be placed at an electric potential optimum for the investigation of the specimen surface with the assistance of the voltage source via which the specimen current flows to ground.
Given specimens having poor conductivity, for example given insulators (oxides, nitrides, glasses, etc), the specimen current deriving from the ion beam current and the charged secondary particle currents emitted by the specimen can only flow off via the very-high electrical resistance of the specimen or, respectively of the specimen surface itself. Due to charging effects on the area of the specimen struck by the ion beam, extreme shifts in potentials can arise, these potential shifts potentially inhibiting or completely preventing the emission and/or the transfer of secondary ions into the mass spectrometer.
When the ion beam consists of positively charged ions, then the surface charging will frequently also be positive and can then be compensated, in principle, by simultaneous bombardment with electrons. It is known from the art to bombard the specimen with a standing electron beam which has a relatively large diameter of its focal spot on the order of about 1 millimeters. As a result thereof, it is usually possible to register secondary ion signals from insulator surfaces.
This known method for the compensation of charges in secondary ion mass spectrometry, however, no longer meets the growing requirements made of the quality, for example of the reproducibility of the test results. The reason for this is that the shape and the point of incidence of the ion beam and of the electron beam deviate too greatly from one another. As a consequence thereof, the electron current density at the various points of incidence of the ion beam during its scan pass is not constant and the uniform compensation of charges for the entire sputter area on the specimen is therefore not achieved.
A further deficiency of this procedure is that negative charges are generated at all locations impinged by the electron beam but not currently impinged by the ion beam, the field of the negative charges having a negative effect on the transfer of the secondary ions triggered later on by the ion beam into the mass spectrometer and therefore falsifying the test result.
In addition, finally, given this known method for the compensation of charges in secondary ion mass spectrometry is the disadvantage that the inherently unnecessary and undesired electron irradiation of those zones on the surface of the specimen momentarily not impinged by the ion beam, these having to be sputtered with the assistance of the ion beam, can induce undesired effect of specimen modifications such as, for example, diffusion and desorption.